Optical recording/playback of information has been made possible by developments in the areas of lasers and thermal record media. Recent developments have led to mass data storage systems utilizing a plurality of individually modulated laser beams to record information at extremely high data rates. For example, in U.S. Pat. No. 4,449,212, issued on May 5, 1984, in the name of the instant inventor, a multi-track record/playback apparatus is described. In the multi-track apparatus the light beam from a single high power laser is split into a plurality of beams which are individually modulated and focused onto the surface of a record medium. In general, systems of this type require large, high power lasers which require external cooling. Furthermore, in these systems a modulator is provided to individually modulate each beam of the multiple beams being used for recording. For these reasons, prior art multi-beam systems tend to be bulky, low in efficiency, and difficult to modulate.
The recent introduction of semiconductor laser arrays has led to the development of multi-channel optical recorders/players which overcome some of the problems of the prior art multi-channel systems. A diode laser array system is generally more compact, has higher efficiency, and requires no external modulation.
In U.S. Pat. No. 4,520,472, issued May 28, 1985, in the name of the instant inventor, an optical system is described for use in a multi-channel record/playback system. The optical system includes a laser diode array and an optical head comprising a collection objective, an anamorphic beam expander, a relay lens and a focusing lens having a finite conjugate. The optical head collects the laser beams emitted by the diode array, expands the beam cross-section to form circular beams and focuses the beams to diffraction limited spots. The relay lens images the lasing spots from the laser diode array in the conjugate plane of the finite conjugate focusing lens.
In a multi-channel system of the Reno ('472) type, the beam expander, which may typically comprise two prisms, compensates for the elliptical character of the laser diode beams, reshaping their emission patterns into symmetrical beams, i.e., circular beams, so that the highest possible spot intensity can be obtained at the recording surface. It has been common practice to direct the central beam expander at an angle equal to the Brewster angle.
When a beam of linearly polarized light is incident on the surface bounding two transparent media of different refraction indexes such that the incident beam vibrates in the plane of incidence, there exists a certain angle of incidence for which the intensity of the reflection beam is equal to zero. That angle .phi. is called the polarization angle, or Brewster angle, and is related to the indexes of refraction of the two media, n.sub.1 and n.sub.2, by the expression EQU .phi.=tan.sup.-1 (n.sub.2 /n.sub.1).
It is noted that when the central beam of a multi-beam diode array, e.g., an array of nine beams, is incident on a prism beam expander at the Brewster angle, there is a variation in the magnification ratios which may be 5 to 6 percent between the least and the greatest. These variations manifest themselves in an optical recording system as variations in track spacing on the recording surface. Since the smallest beam spacing of the array must determine the recording track pitch, the 5 to 6 percent variation across the array produces inefficiency in the recording surface utilization.